Digital black clamp circuit in electronic imaging systems

Abstract

The present invention is a black clamp circuit for an electronic imager having black reference pixels. The circuit includes an average value circuit determining an average of a number of the black reference pixels, a correction circuit determining the black level correction from the average, and a modifier circuit modifying image pixels with the black level. The average value is used in a real time feedback loop that removes offset errors in the signal processing chain and establishes a corrected black level in output image data. The feedback can be either digital or analog. The correction circuit can include a comparator that allows a hysteresis in the feedback to avoid hunting as well as multiple levels of feedback based on several thresholds. The corrected black level can also be controlled by a digital transfer function. The black clamp circuit can be used for a series of images, an image frame and an image line.

Description

FIELD OF THE INVENTION[0001]The invention relates generally to the field of imaging, and in particular to black level clamping in digital sensor imaging. More specifically, the invention relates to a black clamp circuit that tracks the changes in black level (due to noise and dark current) and removes errors in the signal processing and does so in real time.BACKGROUND OF THE INVENTION[0002]Typically, a border of an image-sensing array, such as a Charge Coupled Device (CCD) imager or a CMOS imager, is surrounded with a number of rows and columns of light shielded, or black, pixels. These pixels provide black reference information or black pixel data to stabilize downstream image processing and establish the correct value for black in the output image. Charged Coupled Device Imager (CCD) and CMOS Imager signal processing chains have a need to establish and maintain the black reference in the signal processing chain.[0003]The image-sensing array black pixel data contains a dark current...

AI Technical Summary

Benefits of technology

[0021]1. This invention operates independent of clock rate and will generate superior quality results as compared to prior art analog solutions to deriving the dark level. The solution of the invention finds the absolute digital average of the dark pixels (plus or minus one least significant bit, digital uncertainty) whereas the analog solution integrates the dark pixel values on a capacitor. This analog solution can only be optimized for one clock rate.

[0022]2. This invention uses a true average value for the derived black reference.

[0023]3. The quantity of pixels to be averaged can be made programmable. The noisier the black signal, the more pixels need to be averaged.

[0024]4. The final black clamp value can be digitally adjusted by changing the magnitude of comparator thresholds, or the transfer function in the look up table. This provides the additional advantage of not truncating the black field, (clipping the valleys in the black image) and providing numerical under range for down stream digital image processing.

[0026]6. This invention can be made to operate as a frame rate clamp and a line rate clamp.

[0027]7. This invention can be integrated with a CMOS Imager.

Problems solved by technology

As the imager is operated, it heats up, due to its' internal power dissipation.

This solution also consumes the power to run the microprocessor and the microprocessor machine cycles which are not available for other functions.

The first stage black clamp does not correct for residual errors that can be caused by the PGA and also does not correct for errors that are caused by the analog-to-digital (A / D) converter circuit appearing at a later stage.

There are usually residual offset errors at the output of the PGA.

These can be due to the PGA itself, and the first stage black clamp not performing “perfectly.” There may be additional offset errors, which accumulate, introduced by each succeeding stage in the signal processing.

For example, the analog-to-digital converter may have offset errors.

This is a very tight requirement.

It also uses valuable processor time.

This consumes power and time.

Images